Determining efficiency of an optical signal source in distributed communication systems

ABSTRACT

Components, systems, and methods for determining efficiency of an optical signal source in distributed communication systems are disclosed. Environmentally induced variations in the performance of optical sources used to convert electrical signals to optical signals (such as laser diodes) at the transmitters within the system can be evaluated in real time. Steps can be taken to compensate for these variations. The efficiency of the laser diode can be measured and provided to receivers in the distributed communication system. The receiver may use information related to the slope efficiency measurement to adjust the gain of the receiving amplifiers to provide desired adjustments to the gain. Thus, the receivers in the remote units (RU) receive information about the slope efficiency of the laser diodes at the head end equipment (HEE) and the HEE receives information about the slope efficiency of the laser diodes at the RU.

PRIORITY APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/059,398, filed Mar. 3, 2016, which is continuation of InternationalApplication No. PCT/IL2014/050844 filed on Sep. 22, 2014, which claimsthe benefit of priority to U.S. Provisional Application No. 61/884,454,filed on Sep. 30, 2013, these applications being incorporated herein byreference.

BACKGROUND

The technology of the disclosure generally relates to managing opticalsources in distributed communication systems and more particularly tomanaging laser diodes.

Wireless communication is rapidly growing, with ever-increasing demandsfor high-speed mobile data communication. As an example, so-called“wireless fidelity” or “WiFi” systems and wireless local area networks(WLANs) are being deployed in many different types of areas (e.g.,coffee shops, airports, libraries, etc.). Distributed communicationsystems (one type of which is a distributed antenna system) communicatewith wireless devices called “clients,” which must reside within thewireless range or “cell coverage area” to communicate with an accesspoint device.

One approach to deploying a distributed antenna system (DAS) involvesthe use of radio frequency (RF) antenna coverage areas, also referred toas “antenna coverage areas.” Antenna coverage areas typically have aradius in the range from a few meters up to twenty meters. Combining anumber of access point devices creates an array of antenna coverageareas. Because the antenna coverage areas each cover small areas, thereare typically only a few users (clients) per antenna coverage area. Thisarrangement allows for minimizing the amount of RF bandwidth sharedamong the wireless system users.

One type of DAS distributes RF communication signals over opticalfibers. A DAS can include head end equipment (HEE) optically coupled toremote units (RUs) or remote antenna units (RAUs) having an antenna toprovide antenna coverage areas. The RUs have RF transceivers coupled toone or more antennas to wirelessly transmit RF communication signals.The antennas in the RUs also receive RF signals from clients in theantenna coverage area which are sent over optical fiber to the HEE.

Optical signals are placed onto the optical fibers by laser diodes suchas the QF9550CM1 Quantum Cascade Laser sold by THORLABS of Newton, N.J.The optical power of the laser diode is proportional to the electricalcurrent that drives the laser diode. System designers expect the powerreceived at the RUs to fall within a certain band, for example, 14 dBmto 17 dBm. To maintain the received power within the desired range, theoverall gain of the system is estimated during system setup by injectinga known signal at the transmitter and measuring the signal at thereceiver. The measured value is returned to the transmitter and anestimate of the overall gain is stored. Based on the overall gain of thesystem, the power level needed at the laser diode may be calculated.While such calibration provides a good first order estimate of theoverall gain of the system, environmental factors may cause variationsfor which the calibration does not compensate. Accordingly, thereremains a need to improve the model of the system and provide theappropriate gain.

No admission is made that any reference cited herein constitutes priorart. Applicant expressly reserves the right to challenge the accuracyand pertinency of any cited documents.

SUMMARY

Embodiments disclosed in the detailed description include components,systems, and methods for determining efficiency of an optical signalsource in distributed communication systems. In embodiments disclosedherein, environmentally induced variations in the performance of opticalsources used to convert electrical signals to optical signals (such aslaser diodes) at the transmitters within the system can be evaluated inreal time. Steps can be taken to compensate for these variations. Inparticular, the efficiency of the laser diode can be measured andprovided to receivers in the distributed communication system. Forexample, the efficiency may be determined as a function of the slope ofthe diode's input current versus power output, i.e., gain. The receivermay use information related to the slope efficiency measurement toadjust the gain of the receiving amplifiers to provide desiredadjustments to the gain. Thus, the receivers in the remote units (RU)receive information about the slope efficiency of the laser diodes atthe head end equipment (HEE) and the HEE receives information about theslope efficiency of the laser diodes at the RU.

One embodiment of the disclosure relates to a distributed antenna system(DAS). The DAS includes head end equipment (HEE). The HEE includes adownlink transmitter comprising a downlink laser diode and an uplinkreceiver configured to receive signals from a plurality of remote units(RUs), and at least one optical fiber coupling the HEE to the pluralityof RUs. Each RU includes one or more antennas configured to communicatewirelessly with one or more remote clients. Each RU also includes adownlink receiver communicatively coupled to the downlink transmitterthrough the optical fiber, and an uplink transmitter communicativelycoupled to the uplink receiver through the optical fiber, wherein theuplink transmitter comprises an uplink laser diode. The DAS alsoincludes a control system associated with one of the HEE or one of theplurality of RUs and configured to determine a slope efficiency of therespective laser diode and provide information related to the slopeefficiency to the receiver associated with the respective laser diode.

An additional embodiment of the disclosure relates to a distributedcommunication system in which the HEE includes a downlink transmittercomprising a downlink laser diode. The HEE also includes an uplinkreceiver configured to receive signals from a plurality of RUs. The HEEalso includes a control system configured to determine a slopeefficiency of the downlink laser diode and provide information relatedto the slope efficiency to one of the plurality of RUs.

An additional embodiment relates to a distributed communication systemhaving an RU configured to communicate wirelessly with one or moreremote clients. The RU includes a downlink receiver configured toreceive signals from HEE through at least one optical fiber, and anuplink transmitter communicatively coupled to an uplink receiver throughthe optical fiber, wherein the uplink transmitter comprises an uplinklaser diode. The RU also includes a control system configured todetermine a slope efficiency of the uplink laser diode and provideinformation related to the slope efficiency to the HEE.

An additional embodiment of the disclosure relates to a method ofcompensating for variations in laser diode performance in a distributedcommunication system. The method includes determining a slope efficiencyof a transmitter's laser diode, providing information relating to theslope efficiency to a receiver in the distributed communication system,and adjusting a link gain at the receiver based on the informationrelating to the slope efficiency.

An additional embodiment of the disclosure relates to a non-transitorycomputer readable medium comprising software with instructions. Theinstructions include determining a slope efficiency of a transmitter'slaser diode, providing information relating to the slope efficiency to areceiver in the distributed communication system, and adjusting a linkgain at the receiver based on the information relating to the slopeefficiency.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theembodiments as described herein, including the detailed description, theclaims, as well as the appended drawings.

The foregoing general description and the following detailed descriptionare merely exemplary, and are intended to provide an overview orframework to understand the nature and character of the claims. Thedrawings are included to provide a further understanding, and areincorporated in and constitute a part of this specification. Thedrawings illustrate one or more embodiment(s), and together with thedescription serve to explain principles and operation of the variousembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary distributed antenna system(DAS);

FIG. 2A is a partially schematic cut-away diagram of an exemplarybuilding infrastructure in which the DAS in FIG. 1 can be employed;

FIG. 2B is an alternative diagram of the DAS in FIG. 2A;

FIG. 3 is a graph showing how laser diode output power varies withtemperature and current;

FIG. 4 is a simplified schematic diagram of a transmitter with anoptical source whose slope efficiency is calculated and controlledaccording to an exemplary embodiment;

FIG. 5 is a simplified schematic diagram of a transmitter and receiverpair according to an exemplary embodiment;

FIG. 6A is a flow chart illustrating an exemplary process of slopeefficiency calculation and optical signal source control according to anexemplary embodiment;

FIG. 6B is a flow chart illustrating an exemplary process of slopeefficiency calculation and optical signal source control according to analternate embodiment; and

FIG. 7 is a schematic diagram of a generalized representation of anexemplary computer system that can be included in the distributedcommunication systems disclosed herein.

DETAILED DESCRIPTION

While the concepts of the present disclosure are applicable to differenttypes of distributed communication systems, an exemplary embodiment of asystem for determining efficiency of an optical signal source is used ina distributed antenna system (DAS) and this exemplary embodiment isexplored herein. Before discussing the efficiency determinationprocesses of the present disclosure starting at FIG. 3, an exemplary DAScapable of distributing radio frequency (RF) communication signals todistributed or remote units (RUs) are first described with regard toFIGS. 1-2A. It should be appreciated that in an exemplary embodiment theRUs may contain antennas such that the RU is a remote antenna unit andmay be referred to as a RAU.

In this regard, the DAS in FIGS. 1-2A can include transmitters locatedin the head end unit or the RU. Embodiments of optical signal sourcecontrol systems in a distributed communication system, including thedistributed antenna systems in FIGS. 1-2A, begin with FIG. 3. The DASsin FIGS. 1-2A discussed below include distribution of RF communicationsignals; however, the DAS are not limited to distribution of RFcommunication signals. Also note that while the DAS in FIGS. 1-2Adiscussed below include distribution of communication signals overoptical fiber, these DAS are not limited to distribution strictly overoptical fiber. Distribution mediums could also include, but are notlimited to, hybrid optical/copper cables. Also, any combination can beemployed that also involves optical fiber for portions of the DAS.

In this regard, FIG. 1 is a schematic diagram of an embodiment of a DAS.In this embodiment, the system is an optical fiber-based DAS 10. The DAS10 is configured to create one or more antenna coverage areas forestablishing communication with wireless client devices located in theRF range of the antenna coverage areas. The DAS 10 provides RFcommunication services (e.g., cellular services). In this embodiment,the DAS 10 includes head-end equipment (HEE) 12 such as a head-end unit(HEU), one or more RUs 14, and an optical fiber 16 that opticallycouples the HEE 12 to the RU 14. The RU 14 is a type of remotecommunication unit. In general, a remote communication unit can supportwireless communication, wired communication, or both. The RU 14 cansupport wireless communication and may also support wired communicationthrough wired service port 40. The HEE 12 is configured to receivecommunication over downlink electrical RF signals 18D from a source orsources, such as a network or carrier as examples, and provide suchcommunication to the RU 14. The HEE 12 is also configured to returncommunication received from the RU 14, via uplink electrical RF signals18U, back to the source or sources. In this regard in this embodiment,the optical fiber 16 includes at least one downlink optical fiber 16D tocarry signals communicated from the HEE 12 to the RU 14 and at least oneuplink optical fiber 16U to carry signals communicated from the RU 14back to the HEE 12.

One downlink optical fiber 16D and one uplink optical fiber 16U could beprovided to support multiple channels each using wave-divisionmultiplexing (WDM), as discussed in U.S. patent application Ser. No.12/892,424 entitled “Providing Digital Data Services in OpticalFiber-based Distributed Radio Frequency (RF) Communication Systems, AndRelated Components and Methods,” incorporated herein by reference in itsentirety. Other options for WDM and frequency-division multiplexing(FDM) are disclosed in U.S. patent application Ser. No. 12/892,424, anyof which can be employed in any of the embodiments disclosed herein.Further, U.S. patent application Ser. No. 12/892,424 also disclosesdistributed digital data communication signals in a DAS which may alsobe distributed in the optical fiber-based DAS 10 either in conjunctionwith RF communication signals or not.

The optical fiber-based DAS 10 has an antenna coverage area 20 that canbe disposed about the RU 14. The antenna coverage area 20 of the RU 14forms an RF coverage area 38. The HEE 12 is adapted to perform or tofacilitate any one of a number of Radio-over-Fiber (RoF) applications,such as RF identification (RFID), wireless local-area network (WLAN)communication, or cellular phone service. Shown within the antennacoverage area 20 is a client device 24 in the form of a mobile device asan example, which may be a cellular telephone as an example. The clientdevice 24 can be any device that is capable of receiving RFcommunication signals. The client device 24 includes an antenna 26(e.g., a wireless card) adapted to receive and/or send electromagneticRF signals.

With continuing reference to FIG. 1, to communicate the electrical RFsignals over the downlink optical fiber 16D to the RU 14, to in turn becommunicated to the client device 24 in the antenna coverage area 20formed by the RU 14, the HEE 12 includes a radio interface in the formof an electrical-to-optical (E/O) converter 28, such as a laser diode.The E/O converter 28 converts the downlink electrical RF signals 18D todownlink optical RF signals 22D to be communicated over the downlinkoptical fiber 16D. The RU 14 includes a receiver with anoptical-to-electrical (O/E) converter 30, such as a photo diode, toconvert received downlink optical RF signals 22D back to electrical RFsignals to be communicated wirelessly through an antenna 32 of the RU 14to client devices 24 located in the antenna coverage area 20.

Similarly, the antenna 32 is also configured to receive wireless RFcommunication from client devices 24 in the antenna coverage area 20. Inthis regard, the antenna 32 receives wireless RF communication fromclient devices 24 and communicates electrical RF signals representingthe wireless RF communication to an E/O converter 34, such as a laserdiode, in the RU 14. The E/O converter 34 converts the electrical RFsignals into uplink optical RF signals 22U to be communicated over theuplink optical fiber 16U. A receiver with an O/E converter 36 providedin the HEE 12 converts the uplink optical RF signals 22U into uplinkelectrical RF signals, which can then be communicated as uplinkelectrical RF signals 18U back to a network or other source.

For both the HEE 12 and the RU 14, the power received at the 0/Econverter 30, 36 should be between 14 dBm and 17 dBm. Typically the E/Oconverters 28, 34 are driven near saturation in an attempt to achievethe desired received power. However, loss in the optical path or otherloss may cause the overall gain of the system to be such that thedesired power levels cannot be achieved. Accordingly, the receivers mayinclude one or more chained amplifiers that boost the incoming signal tothe desired levels. More information on the receivers is provided belowwith reference to FIG. 4.

To provide further exemplary illustration of how a distributed antennasystem can be deployed indoors, FIG. 2A is provided. FIG. 2A is apartially schematic cut-away diagram of a building infrastructure 50employing an optical fiber-based DAS. The system may be the opticalfiber-based DAS 10 of FIG. 1. The building infrastructure 50 generallyrepresents any type of building in which the optical fiber-based DAS 10can be deployed. As previously discussed with regard to FIG. 1, theoptical fiber-based DAS 10 incorporates the HEE 12 to provide varioustypes of communication services to coverage areas within the buildinginfrastructure 50.

For example, as discussed in more detail below, the DAS 10 in thisembodiment is configured to receive wireless RF signals and convert theRF signals into RoF signals to be communicated over the optical fiber 16to multiple RUs 14. The optical fiber-based DAS 10 in this embodimentcan be, for example, an indoor distributed antenna system (IDAS) toprovide wireless service inside the building infrastructure 50. Thesewireless signals can include cellular service, wireless services such asRFID tracking, Wireless Fidelity (WiFi), local area network (LAN), WLAN,public safety, wireless building automations, and combinations thereof,as examples.

With continuing reference to FIG. 2A, the building infrastructure 50 inthis embodiment includes a first (ground) floor 52, a second floor 54,and a third floor 56. The floors 52, 54, 56 are serviced by the HEE 12through a main distribution frame 58 to provide antenna coverage areas60 in the building infrastructure 50. Only the ceilings of the floors52, 54, 56 are shown in FIG. 2A for simplicity of illustration. In theexample embodiment, a main cable 62 has a number of different sectionsthat facilitate the placement of a large number of RUs 14 in thebuilding infrastructure 50. Each RU 14 in turn services its own coveragearea in the antenna coverage areas 60. The main cable 62 can include,for example, a riser cable 64 that carries all of the downlink anduplink optical fibers 16D, 16U to and from the HEE 12. The riser cable64 may be routed through a power unit 70. The power unit 70 may also beconfigured to provide power to the RUs 14 via an electrical power lineprovided inside an array cable 72, or tail cable or home-run tethercable as other examples, and distributed with the downlink and uplinkoptical fibers 16D, 16U to the RUs 14. For example, as illustrated inthe building infrastructure 50 in FIG. 2B, a tail cable 80 may extendfrom the power units 70 into an array cable 82. Downlink and uplinkoptical fibers in tether cables 84 of the array cables 82 are routed toeach of the RUs 14, as illustrated in FIG. 2B. Referring back to FIG.2A, the main cable 62 can include one or more multi-cable (MC)connectors adapted to connect select downlink and uplink optical fibers16D, 16U, along with an electrical power line, to a number of opticalfiber cables 66.

With continued reference to FIG. 2A, the main cable 62 enables multipleoptical fiber cables 66 to be distributed throughout the buildinginfrastructure 50 (e.g., fixed to the ceilings or other support surfacesof each floor 52, 54, 56) to provide the antenna coverage areas 60 forthe first, second, and third floors 52, 54, and 56. In an exampleembodiment, the HEE 12 is located within the building infrastructure 50(e.g., in a closet or control room), while in another exampleembodiment, the HEE 12 may be located outside of the buildinginfrastructure 50 at a remote location. A base transceiver station (BTS)68, which may be provided by a second party such as a cellular serviceprovider, is connected to the HEE 12, and can be co-located or locatedremotely from the HEE 12. A BTS (such as BTS 68) is any station orsignal source that provides an input signal to the HEE 12 and canreceive a return signal from the HEE 12.

In a typical cellular system, for example, a plurality of BTSs isdeployed at a plurality of remote locations to provide wirelesstelephone coverage. Each BTS serves a corresponding cell and when amobile client device enters the cell, the BTS communicates with themobile client device. Each BTS can include at least one radiotransceiver for enabling communication with one or more subscriber unitsoperating within the associated cell. As another example, wirelessrepeaters or bi-directional amplifiers could also be used to serve acorresponding cell in lieu of a BTS. Alternatively, radio input could beprovided by a repeater, picocell, or femtocell.

The optical fiber-based DAS 10 in FIGS. 1-2B and described aboveprovides point-to-point communication between the HEE 12 and the RU 14.A multi-point architecture is also possible as well. With regard toFIGS. 1-2B, each RU 14 communicates with the HEE 12 over a distinctdownlink and uplink optical fiber pair to provide the point-to-pointcommunication. Whenever an RU 14 is installed in the optical fiber-basedDAS 10, the RU 14 is connected to a distinct downlink and uplink opticalfiber pair connected to the HEE 12. The downlink and uplink opticalfibers 16D, 16U may be provided in a fiber optic cable. Multipledownlink and uplink optical fiber pairs can be provided in a fiber opticcable to service multiple RUs 14 from a common fiber optic cable.

For example, with reference to FIG. 2A, RUs 14 installed on a givenfloor 52, 54, or 56 may be serviced from the same optical fiber 16. Inthis regard, the optical fiber 16 may have multiple nodes where distinctdownlink and uplink optical fiber pairs can be connected to a given RU14.

The HEE 12 may be configured to support any frequencies desired,including but not limited to US FCC and Industry Canada frequencies(824-849 MHz on uplink and 869-894 MHz on downlink), US FCC and IndustryCanada frequencies (1850-1915 MHz on uplink and 1930-1995 MHz ondownlink), US FCC and Industry Canada frequencies (1710-1755 MHz onuplink and 2110-2155 MHz on downlink), US FCC frequencies (698-716 MHzand 776-787 MHz on uplink and 728-746 MHz on downlink), EU R & TTEfrequencies (880-915 MHz on uplink and 925-960 MHz on downlink), EU R &TTE frequencies (1710-1785 MHz on uplink and 1805-1880 MHz on downlink),EU R & TTE frequencies (1920-1980 MHz on uplink and 2110-2170 MHz ondownlink), US FCC frequencies (806-824 MHz on uplink and 851-869 MHz ondownlink), US FCC frequencies (896-901 MHz on uplink and 929-941 MHz ondownlink), US FCC frequencies (793-805 MHz on uplink and 763-775 MHz ondownlink), and US FCC frequencies (2495-2690 MHz on uplink anddownlink).

As noted above, both the HEE 12 and the RU 14 have E/O converters 28,34, which typically include laser diodes. Laser diodes convert anelectrical current into a beam of coherent light that may be used toprovide optical signals. These laser diodes are designed to operatelinearly over a range of currents allowing system designers to have ageneral idea of output power for the laser diode. The estimated outputpower may be used in designing the system (e.g., how long maycommunication paths before a repeater is needed). That is, the outputpower of the laser diode is designed to be proportional to the currentsupplied to the laser diode. However, most laser diodes are temperaturesensitive such that they provide different output powers for the samecurrent at different temperatures. It is this temperature inducedvariation that may cause the overall system gain to differ from a DAS,including the DAS 10 in FIG. 1, system gain approximated during systeminstallation calibration.

In this regard, FIG. 3 is an exemplary graph 90 depicting the gain of alaser diode for two different temperatures. In particular, graph 90shows the output power 92 (y-axis) versus current 94 (x-axis) for alaser diode at a first temperature 96 (e.g., 25° C.) and a secondtemperature 98 (e.g., 50° C.). As can readily be seen, a given outputpower P_(OPT1) may require a first current I_(LD1) at first temperature96, but a second current I_(LD3) at second temperature 98. Likewise, asecond output power P_(OPT2) may require a first current I_(LD2) atfirst temperature 96 and a second current I_(LD4) at second temperature98. In theory, to get a desired output power, the current provided tothe laser diode might be varied as the temperature varied. For example,if P_(OPT2) was a desired output power, the current supplied to thelaser diode could change from I_(LD2) to I_(LD4) if the temperatureincreased from first temperature 96 to second temperature 98. However,in practice, power supply constraints may preclude such variations incurrent to the laser diode, or such variations in the current may takethe laser diode out of its linear operating range. For at least thesereasons, the simple activity of varying current to the laser diode isimpractical.

The present disclosure accepts that variations in the current are notdesired and instead provides information to a receiver about thevariations in the gain of the laser diode. The DAS receiver can thencompensate for the gain variations as appropriate. Exemplary structuresassociated with embodiments of the present disclosure are set forth withreference to FIGS. 4 and 5, which are discussed below.

In this regard, FIG. 4 illustrates a transmitter 100 that may bepositioned in either the HEE 12 or the RU 14. The transmitter 100includes a control system (described in greater detail below) configuredto calculate a slope efficiency of an optical source and provide controlsignals based on the calculated slope efficiency. In particular, thetransmitter 100 includes an RF input 102 that receives an RF signal 104.The RF signal 104 is amplified by an amplifier 106. A capacitor 108blocks direct current (DC) signals from node 110. Node 110 takes theamplified signal from the amplifier 106 and a direct current signal fromcurrent source 112 and sums the signals into a summed signal which ispresented to a laser diode 114. The laser diode 114 converts the currentreceived to an optical signal 116 which is passed to optical fiber 118,which is analogous to optical fiber 16 of FIGS. 1-2B. While the bulk ofthe light emitted by the laser diode 114 is transmitted through theoptical fiber 118, some portion is detected by photodiode (PD) 120 whichgenerates a voltage (VPD) at node 122 across resistor 124. VPD isprovided to a control system 126. Additionally a temperature sensor 128may be associated with the transmitter 100 to measure temperature.Temperature sensor 128 is also coupled to the control system 126. Thecontrol system 126 may use data from the photodiode 120 and/ortemperature sensor 128 to generate a management signal 129 that may becombined with the RF signal 104 and transmitted over the optical fiber118. Further, the control system 126 provides a control signal to thecurrent source 112 that instructs the current source to provide aparticular direct current (DC) signal at node 110. Exemplary processesthrough which the transmitter 100 calculates the gain of the laser diode114 are set forth below with reference to FIGS. 6A and 6B.

FIG. 5 is a simplified view of a distributed communication system 130with a transmitter 100 and a receiver 132. The transmitter 100 isdiscussed above with reference to FIG. 4 and that discussion is notrepeated. The receiver 132 includes a photodiode 134 (i.e., 0/Econverter 30 or 34). The output of the photodiode 134 is provided to anamplifier chain 136 (only one illustrated). The amplifier chain 136provides an output RF signal 138 at output node 140. The amplifier chain136 compensates for the overall gain of the system. In general, thelater compensation occurs in the amplifier chain 136, the less optimalthe compensation is. That is, if, for example, there are four amplifiersin the amplifier chain 136, and the compensation occurs at the third orfourth amplifier, the compensation may be distorted by the previouslyapplied amplification of the first or second amplifier. Accordingly, itis desirable to apply the compensation earlier in the amplifier chain136.

In this regard, the present disclosure provides techniques that allowfor the gain of the optical signal source (i.e., the laser diode) to becalculated in real time. Such real time calculations do not require thatthe DAS discontinue normal operations. Further, such real timecalculations allow effectively instantaneous corrections forenvironmental factors such as temperature which affect the gain of theDAS.

Against the backdrop of the elements presented in FIGS. 4 and 5,processes for calculating slope efficiency and control of the opticalsignal source according to exemplary embodiments of the presentdisclosure are provided with reference to FIGS. 6A and 6B. In thisregard, FIG. 6A illustrates a first exemplary process 150. The process150 begins with the initial installation of a distributed communicationsystem 10 such as a DAS (block 152). The installation concludes with theoverall system calibration of the overall system gain (block 154).Normal operations begin (block 156) with communication signals sent toand received from remote clients through the RU 14 and passed to the BTS68 as is well understood.

The control system 126 either sets or has set a period for gaincalculation (block 158). The period for gain calculation may be once asecond, once a minute, or other period as needed or desired. However,environmental conditions do not typically change particularly quicklyand a relatively slow period may be appropriate. The period may be setby the installation personnel, preprogrammed into the software and/orfirmware of the control system during construction, or provided at alater time as needed or desired. Once the period is set and normaloperations are active, the control system 126 monitors to determine ifthe period has expired (block 160). If the period has not expired,monitoring continues as noted. If, the period has expired, the controlsystem 126 instructs the current source 112 to provide a first DCcurrent I_(LD1) to the node 110 and thus to the laser diode 114 (block162). In an exemplary embodiment, the first DC current I_(LD1) isapproximately five milliamps (mA).

The addition of the DC current I_(LD1) causes a change in the outputpower of the laser diode 114. Photodiode 120 receives some of the energyemitted by the laser diode 114 and generates a voltage V_(PD1) acrossthe resistor 124. The output voltage V_(PD1) is measured (block 164).The control system 126 causes the current source 112 to provide a secondDC current I_(LD2) (block 166). In an exemplary embodiment, the secondDC current I_(LD2) is approximately ten mA. Other current levels may beselected as needed or desired, but a spread of five mA between I_(LD1)and I_(LD2) provides reliably distinct measurements. The output voltageV_(PD2) is measured across the resistor 124 (block 168). From the outputvoltages V_(PD1) and V_(PD2), corresponding output power levels P_(OPT1)and P_(OPT2) may be calculated (block 170). With the output power levelsP_(OPT1) and P_(OPT2) and input current levels I_(LD1) and I_(LD2), thecontrol system 126 may calculate a slope efficiency (SE) (block 172)through the equation:

${SE} = \frac{P_{{OPT}\; 2} - P_{{OPT}\; 1}}{I_{{LD}\; 2} - I_{{LD}\; 1}}$

SE is an effective measurement of the gain of the laser diode 114. Thus,changes in SE are changes in the gain of the laser diode 114. It isthese environmentally induced changes in the gain of the laser diode 114that affect the overall system gain and are not considered during theinitial calibration of the system. Accordingly, once the control system126 has the SE, the control system 126 can provide information relatedto the SE in the management signal 129 to the receiver 132.

In an alternate embodiment, instead of periodically calculating the SE,the SE is only calculated when the temperature has changed beyond apredefined threshold. In an exemplary embodiment, the threshold may be2° C. Other thresholds may be used as needed or desired. FIG. 6Billustrates such alternate process 180. Process 180 is substantiallysimilar to process 150, and many of the steps are not repeated. However,in relevant part, process 180 measures the temperature with sensor 128(block 182). Measurements may be taken periodically or continuously. Thecontrol system 126 determines if the temperature has changed more thanthe predefined threshold (block 184). If the temperature has not changedsufficiently, the temperature sensor 128 continues to monitor. If thetemperature has changed more than the threshold, then the control system126 calculates SE as explained above with reference to FIG. 6A.

The processes 150 and 180 offer some tradeoffs. The use of thetemperature sensor 128 requires the additional hardware of thetemperature sensor 128 as well as the inputs for the control system 126to interoperate with the temperature sensor 128. In contrast, theperiodic calculations of process 150 may unnecessarily consume power ifSE has not changed substantially. Such tradeoffs may be evaluated duringsystem design.

FIG. 7 is a schematic diagram representation of additional detailregarding an exemplary computer system 200 that may be included in theHEE 12 or the RU 14. The computer system 200 is adapted to executeinstructions from an exemplary computer-readable medium to perform powermanagement functions. In this regard, the computer system 200 mayinclude a set of instructions for causing the control system 126 tocalculate the SE and send the management signal 129 as previouslydescribed. The HEE 12 or RU 14 may be connected (e.g., networked) toother machines in a LAN, an intranet, an extranet, or the Internet. TheHEE 12 or RU 14 may operate in a client-server network environment, oras a peer machine in a peer-to-peer (or distributed) networkenvironment. While only a single device is illustrated, the term“device” shall also be taken to include any collection of devices thatindividually or jointly execute a set (or multiple sets) of instructionsto perform any one or more of the methodologies discussed herein. Thecontrol system 126 may be a circuit or circuits included in anelectronic board card, such as a printed circuit board (PCB) as anexample, a server, a personal computer, a desktop computer, a laptopcomputer, a personal digital assistant (PDA), a computing pad, a mobiledevice, or any other device, and may represent, for example, a server ora user's computer.

The exemplary computer system 200 in this embodiment includes aprocessing device or processor 202, a main memory 214 (e.g., read-onlymemory (ROM), flash memory, dynamic random access memory (DRAM) such assynchronous DRAM (SDRAM), etc.), and a static memory 206 (e.g., flashmemory, static random access memory (SRAM), etc.), which may communicatewith each other via the data bus 208. Alternatively, the processingdevice 202 may be connected to the main memory 214 and/or static memory206 directly or via some other connectivity means. The processing device202 may be a controller, and the main memory 214 or static memory 206may be any type of memory.

The processing device 202 represents one or more general-purposeprocessing devices such as a microprocessor, central processing unit, orthe like. More particularly, the processing device 202 may be a complexinstruction set computing (CISC) microprocessor, a reduced instructionset computing (RISC) microprocessor, a very long instruction word (VLIW)microprocessor, a processor implementing other instruction sets, orprocessors implementing a combination of instruction sets. Theprocessing device 202 is configured to execute processing logic ininstructions 204 for performing the operations and steps discussedherein.

The computer system 200 may further include a network interface device210. The computer system 200 also may or may not include an input 412 toreceive input and selections to be communicated to the computer system200 when executing instructions. The computer system 200 also may or maynot include an output 222, including but not limited to a display, avideo display unit (e.g., a liquid crystal display (LCD) or a cathoderay tube (CRT)), an alphanumeric input device (e.g., a keyboard), and/ora cursor control device (e.g., a mouse).

The computer system 200 may or may not include a data storage devicethat includes instructions 216 stored in a computer-readable medium 218.The instructions 224 may also reside, completely or at least partially,within the main memory 214 and/or within the processing device 202during execution thereof by the computer system 200, the main memory 214and the processing device 202 also constituting computer-readable medium218. The instructions 216, 224 may further be transmitted or receivedover a network 220 via the network interface device 210.

Further, as used herein, it is intended that the terms “fiber opticcables” and/or “optical fibers” include all types of single mode andmulti-mode light waveguides, including one or more optical fibers thatmay be upcoated, colored, buffered, ribbonized and/or have otherorganizing or protective structure in a cable such as one or more tubes,strength members, jackets or the like. The optical fibers disclosedherein can be single mode or multi-mode optical fibers, bend-insensitiveoptical fibers, or any other expedient of a medium for transmittinglight signals.

Many modifications and other embodiments of the embodiments set forthherein will come to mind to one skilled in the art to which theembodiments pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. For example, the DAScould include any type or number of communication mediums, including butnot limited to electrical conductors, optical fiber, and air (i.e.,wireless transmission).

Unless expressly stated, it is not intended that any method set forthherein be construed as requiring that its steps be performed in aspecific order. Accordingly, where a method claim does not actuallyrecite an order to be followed by its steps or it is not otherwisespecifically stated in the claims or descriptions that the steps are tobe limited to a specific order, no particular order should be inferred.

Various modifications and variations can be made without departing fromthe spirit or scope of the invention. Since modifications combinations,sub-combinations and variations of the disclosed embodimentsincorporating the spirit and substance of the invention may occur topersons skilled in the art, the invention should be construed to includeeverything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A method of compensating for variations in laserdiode performance in a communication system, the method comprising:determining a slope efficiency of a transmitter's laser diode; providinginformation relating to the slope efficiency to a receiver; andadjusting a link gain in a radio frequency (RF) amplifier at thereceiver based on the information relating to the slope efficiency,wherein determining the slope efficiency comprises measuring atemperature at the laser diode and determining the slope efficiency ifthe temperature has changed more than a threshold amount compared to apreviously recorded temperature.
 2. The method of claim 1, whereinadjusting the link gain occurs in substantially real time during activecommunication by the communication system.
 3. The method of claim 2,wherein providing information relating to the slope efficiency to thereceiver in the communication system comprises sending the informationin a management signal.
 4. The method of claim 1, wherein determiningthe slope efficiency comprises periodically determining the slopeefficiency.
 5. The method of claim 4, wherein providing informationrelating to the slope efficiency to the receiver in the communicationsystem comprises sending the information in a management signal.
 6. Amethod of compensating for variations in laser diode performance in acommunication system, the method comprising: determining a slopeefficiency of a transmitter's laser diode; providing informationrelating to the slope efficiency to a receiver; and adjusting a linkgain at the receiver based on the information relating to the slopeefficiency, wherein determining the slope efficiency comprises:measuring a first current level; and measuring a first output power fromthe laser diode at the first current level.
 7. The method of claim 6,wherein adjusting the link gain at the receiver comprises adjusting thelink gain in a radio frequency (RF) amplifier.
 8. The method of claim 7,wherein determining the slope efficiency further comprises: adjusting acurrent source to a second current level; and measuring a second outputpower from the laser diode at the second current level.
 9. The method ofclaim 7, wherein adjusting the link gain occurs in substantially realtime during active communication by the communication system.
 10. Themethod of claim 7, wherein determining the slope efficiency comprisesperiodically determining the slope efficiency.
 11. The method of claim7, wherein providing information relating to the slope efficiency to thereceiver in the communication system comprises sending the informationin a management signal.
 12. A method of compensating for variations inlaser diode performance in a communication system, the methodcomprising: determining a slope efficiency of a transmitter's laserdiode; providing information relating to the slope efficiency to areceiver; and adjusting a link gain at the receiver based on theinformation relating to the slope efficiency, wherein providinginformation relating to the slope efficiency to the receiver in thecommunication system comprises sending the information in a managementsignal.
 13. The method of claim 12, wherein determining the slopeefficiency comprises measuring a temperature at the laser diode anddetermining the slope efficiency if the temperature has changed morethan a threshold amount compared to a previously recorded temperature.14. The method of claim 13, wherein adjusting the link gain occurs insubstantially real time during active communication by the communicationsystem.
 15. The method of claim 13, wherein determining the slopeefficiency comprises periodically determining the slope efficiency. 16.The method of claim 13, wherein adjusting the link gain at the receivercomprises adjusting the link gain in an amplifier.
 17. The method ofclaim 12, wherein determining the slope efficiency comprises: measuringa first current level; measuring a first output power from the laserdiode at the first current level; adjusting a current source to a secondcurrent level; and measuring a second output power from the laser diodeat the second current level.